Impedance matching circuit for an electrodeless fluorescent lamp ballast

Abstract

An impedance matching circuit for a self-oscillating electrodeless fluorescent lamp ballast of the type having an inductor connected in series with the parallel combination of a capacitor and the lamp's drive coil includes an additional capacitor connected in series with the drive coil. The capacitance value chosen for the additional capacitor is dependent on: stresses on the parallel capacitor; matching the impedance of the ballast; and the impact of the capacitor on the loaded coil phase angle. The additional capacitor reduces the phase angle presented to the ballast, thereby lowering the sensitivity of the ballast to component and lamp variations. In addition, the overall impedance of the network is reduced, such that the required inductance of the series inductor is reduced; hence, the inductor can have fewer turns and lower conduction losses. The current in inductor for the required power level is lower, resulting in a further reduction in conduction losses as well as a reduction in core losses due to lower flux in the core. Still further, a reduction of stresses on the inductor results in a reduced operating temperature, and hence increased efficiency, reliability and ballast life.

What is claimed is: 
     
       1. A ballast for an electrodeless of a type of a type having a drive coil situated proximate a light-transmissive envelope for exciting an arc discharge in an ionizable fill contained therein when excited by an alternating current power supply, comprising: at least two switching devices connected in a bridge configuration; and   an impedance matching circuit coupled to said bridge configuration of switching devices, said matching circuit comprising an inductance connected in series with a parallel connection of a parallel matching capacitor and said drive coil, said parallel matching capacitor having a capacitance value selected for matching the resistive portion of the impedance of the ballast, said matching circuit further comprising a series matching capacitor connected in series with said drive coil, said series matching capacitor having a capacitance value selected to optimize the phase angle of the impedence of said ballast so as to avoid sensitivity to ballast component and lamp variations and to minimize said inductance.   
     
     
       2. The ballast of claim 1, comprising an electrodeless fluorescent lamp ballast. 
     
     
       3. An electrodeless fluorescent lamp, comprising: a light-transmissive envelope containing an ionizable, gaseous fill for sustaining an arc discharge when subjected to a radio frequency magnetic field and for emitting ultraviolet radiation as a result thereof, said envelope having an interior phosphor coating for emitting visible radiation when excited by said ultraviolet radiation;   a reflector situated about a reflector potion of said envelope for reflecting said visible radiation through a light-emitting portion of said envelope situated opposite said reflector portion; and   a drive coil situated proximate said envelope for providing said radio frequency magnetic field when excited by a radio frequency energy source via a ballast, said ballast comprising at least two switching devices connected in a bridge configuration and an impedance matching circuit coupled to said bridge configuration of switching devices, said matching circuit comprising an inductance connected in series with a parallel connection of a parallel matching capacitor and said drive coil, said matching circuit further comprising a series matching capacitor connected in series with said drive coil, said series matching capacitor having a capacitance value selected to optimize the phase angle of the impedance of said ballast so as to avoid sensitivity to ballast component and lamp variations and to minimize said inductance.